(a) Technical Field
The present invention relates to a method for suppressing a circulating current in a modular multi-level converter for high voltage direction-current transmission. More specifically, it relates to a method for suppressing a circulating current in a modular multi-level converter for high voltage direction-current transmission, which can further improve a current flowing in a direct current line end while further improving the circulating current suppression characteristics.
(b) Background Art
Generally, the High Voltage Direct-Current (HVDC) transmission has advantages such as the long distance transmission, the asynchronous system connection, the submarine cable use, and the possible power control compared to the High Voltage Alternating-Current (HVAC) transmission, and thus the application cases of HVDC are steadily increasing.
For example, it is known that the HVDC system has lower energy transportation cost than the HVAC for the cases where a see is placed between an energy production location and an energy consumption location or where the distance between the energy production location and the energy consumption location is over 1,000 km
Thus, as the commercialization magnitude is becoming larger about the marine wind power generation, one of the new renewable energy sources and the long-term plans to develop in a large quantity are released, HVDC technologies that are cheap, flexible and stable for transmitting power generated in a marine place to a terrestrial place are drawing attention.
Besides these reasons, the HVDC technologies have been studied for various purposes such as international energy transaction (electric power transaction), energy transaction between different electric power systems having different electric power system frequencies, installation of additional lines at a low cost when energy bottleneck phenomenon occurs due to the extensive energy consumption in downtowns that are densely populated areas.
The HVDC converter is a device that is a core of the HVDC system and is roughly divided into a current type converter and a voltage type converter. The present invention relates to a voltage type converter, and more particularly, to an HVDC converter with a “modular multi-level converter” among the voltage type converter, in which sub-modules (SM) are stacked in series to endure a high voltage.
FIG. 1 is a view illustrating an overall configuration of an HVDC system with a Modular Multilevel Converter (MMC). The Modular Multilevel Converters (MMC-1 and MMC-2) 3 and 4 that constitute the High Voltage Direct Current (HVDC) system transmit energy by the following process. A frequency and a voltage are applied to two different high voltage AC systems 1 and 2 to convert AC energy of one high voltage AC system 1 into DC energy, transmit the DC energy up to a long distance using a DC cable, and then re-convert the arrived DC energy into AC energy having a voltage and a frequency that are suitable for the other high voltage AC system 2.
The modular multilevel converter can generate a high voltage source by in series stacking a plurality of sub-modules 5 with a low voltage source when AC energy is converted into DC energy and vice versa.
The Modular Multilevel Converter (MMC) includes a total of three legs, one for each phase. Here, each leg includes an upper arm 6a and a lower arm 6b, and each arm includes sub-modules 5 that are connected in series.
FIG. 2 is a view illustrating types of sub-modules of a modular multilevel converter for HVDC. Examples of sub-modules include a Half Bridge (HB)-type sub-module, a Full Bridge (FB)-type sub-module, and a Camp Double (CD)-type sub-module. Most of the commercialized HDVC modular multilevel converters are using the HB-type.
The MMC has the following advantages. A sub-module unit is manufactured using an IGBT with a low voltage specification, and the MMC can have a withstand voltage ability with respect to a high voltage of hundreds of KV by stacking the sub-modules in series. Also, a waveform close to a basic wave (sine wave) can be formed using a plurality of sub-modules without a separate filter. Active power control and reactive power control that are known as a limitation of the current-type converter can be independently performed, and there is no need to together supply reactive power which corresponds to 50% of the transmission power for the active power control. Also, each of converters located at the both ends of a high DC voltage can be stably controlled without using the state or information of the counter converter.
However, the HVDC converter with the modular multilevel converter has limitations that are not present in the current-type converter.
In other words, the capacity voltage in the sub-modules is not uniform. Also, since a combined voltage of the upper arm and the lower arm is not the same as a DC_link voltage, a current corresponding to the half of a phase current and a circulating current including an AC component having a frequency two times larger than the system frequency flow in each arm of the multi-level converter. This circulating current flows only in the multilevel converter, and is known to be unnecessary for transmitting electrical energy.
As described above, if the circulating current increases in the HVDC multilevel converter, a current of a reactive component that does not act in the energy conversion additionally flows, and thus the current specifications of the components (IGBT, capacitor, SCR, and fast switch) used in the sub-modules become higher, resulting in an increase of the material cost. Also, since the fluctuation range of the sub-module voltage becomes larger, the sub-module cannot be stably controlled. Also, due to a signal type in which a lot of harmonic components are included in the arm current waveform, the loss of the converter increases, making it difficult to increase the efficiency of the converter.
Hereinafter, a representative method for inhibiting the circulating current component generated in the HVDC converter to which the modular multilevel converter is applied will be described below.
In the modular multilevel converter for the HVDC system, a method of separating and controlling an upper arm and a lower arm for each phase and a method of smoothing a sub module voltage was greatly developed by Antonios Antonopoulos (2009). Also, regarding the unbalanced system voltage condition as well, a current controlling method capable of effectively and quickly controlling the active power (or the DC_link constant voltage control) and the reactive power was developed by Maryam Saeedifard (2010), which is shown in FIG. 3.
On the basis of these technical foundations, various methods have been proposed on the method of suppressing a circulating current generated in the modular multilevel converter. Qingrui Tu (2012) has proposed a method of suppressing a circulating current in both balanced voltage condition and unbalanced system voltage.
FIG. 4 is a schematic view illustrating a method for suppressing a circulating current in a modular multilevel converter for an HVDC system, published by Qingrui Tu [IEEE Trans. on Power Delivery, vol. 27, 2012]. Qingrui Tu (2012) proposed a method for suppressing a circulating current by introducing a method of controlling a negative sequence component and a zero sequence component among the components of the circular current to zero in order to remove the circulating current.
In other words, Qingrui Tu proposed a method of suppressing a circular current by calculating a compensation value v*diffj indicated by A of FIG. 3 in his thesis of 2012.
To see the method proposed by Qingrui Tu in more detail, as shown in FIG. 4, in case of the negative sequence component, the control objectives (command values) in*ccd=0 and in*ccq=0 are given in the d-q frame (d-q rotating coordinate system), and the output of the vnccd and vnccq is controlled through a PI controller 7 so that the component of d-axis (inccd) and the component of q-axis (inccq) are followed among the negative sequence components of the circulating current. Also, the 2-phase is inversely converted into the 3-phase through T−1(2θs) to add to the calculation of the compensation value Vdiff_abc.
Also, in case of the zero sequence component, a method of giving a control objective in the abc frame (3-phase stationary reference frame), adding up all six voltage values upa, una, upb, unb, upc, unb (here, the subscripts p, a, and n indicate upper arm, phase, and lower arm, respectively) of the upper arm and the lower arm in each phase (a, b, c phase), dividing the added value by three, and then outputting Vdiff0_abc through a band pass filter 8 to calculate the compensation value (Vdiff_abc) was applied.
In other words, the method proposed by Qingrui Tu is a method of suppressing a circulating current in consideration of only the negative sequence component and zero sequence component among the components of the circulating current.
Accordingly, in the related-art method for suppressing the circulating current (Qingrui Tu, 2012), it is necessary to separate the component of the circulating current into the positive sequence component, the negative sequence component, and the zero sequence component, and all information on the upper arm voltage and the lower arm voltage need to be known. Also, the method does not include the concept of removing the positive sequence component of the circulating current. Furthermore, the stationary state characteristics are good, but the transient state characteristics are not good and a pulsating current occurs in the direct current line.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.